TUESDAY, May 24 (HealthDay News) -- Humans possess the ability to hear far higher pitched sounds when underwater than they can while on terra firma.
How can they do it? By "hearing" with their bones rather than through the normal pathways of hearing, U.S. Navy researchers report.
The way in which humans hear above or below water differs; that difference means they only hear between 20 and 20,000 hertz through the air, while they can catch sounds all the way up to 200,000 hertz when submerged.
On land, humans hear through air conduction. Sound pressure waves cause tiny disturbances in the air that travel into the ear canal and vibrate the ear drum, which is connected to the three smallest bones in the body, the ossicles of the middle ear.
The ossicles are connected to the cochlea, which is filled with fluid and contains "hair cells," or tiny protrusions that also move, stimulating the auditory nerve, which sends signals to the brain.
"Human hearing is magnificent," said Bill Martin, a hearing scientist at the Oregon Hearing Research Center in Portland. "The ear drum only has to move less than the diameter of a hydrogen atom for us to perceive sound."
The cochlea, too, is complex. Similar to a rolled up piano keyboard, portions of the cochlea are responsible for hearing specific frequencies that help humans differentiate between, say, a bird tweeting and a garbage truck.
But underwater, humans don't hear using the normal channels. Instead, the study found that humans hear through bone conduction, which bypasses the outer ear and the ossicles of the middle ear.
Instead, sound comes through the mastoid, or the bone you can feel if you put your fingers behind the ear.
"By using bone conduction, the human ear can actually receive sounds at frequencies way higher than most people would have expected," Martin explained.
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